Problem: On the first 4 exams of her language class, Tiffany got an average score of 81. What does she need on the next exam to have an overall average of 83?
Solution: Let her score on the next exam be $x$ The sum of all of her scores is then $4 \cdot 81 + x$ The same sum must also be equal to $5 \cdot 83$ Solve: $x = 5 \cdot 83 - 4 \cdot 81 = 91$.